The end of twentieth century is characterized by a large increase of thermoplastic consumption with the consequent increase in their prices [1,2]. This situation has created an impetus for cost reduction by utilizing fillers in thermoplastic to form composite materials [3]. Composite materials also offer the opportunity to utilize the favorable properties of the constituent components [1-31].
The cost of wood-fibers is substantially lower than the other commonly used fibrous filler materials such as glass, mica and the like. Plastic/wood-fiber composites exhibit improved stiffness in addition to their lower costs. They can be a cost effective alternative to many filled plastics or metals in terms of bending stiffness or weight [3]. The wood-fibers are non-abrasive so that relatively large concentrations can be incorporated into plastics without causing serious machine wear during blending and processing.
Although plastic/wood-fiber composites have been commercialized, and can compete with certain plastic and wood products, their potential industrial applications have been limited because of their low impact strength and high density compared to natural wood and certain plastics [4].
Impact strength and ductility of plastics can be significantly improved by incorporating a fine cell structure into them [5]. Additional benefits of having a foamed structure are the reduction in weight and cost. Therefore, it is reasonable to expect that, if a fine-celled structure is successfully produced in plastic/wood-fiber composites, the problems with impact strength and weight mentioned earlier can be reduced or eliminated. This would significantly increase the potential applications of these composites.
Water vapor adsorbed in the wood fiber is released during heating (plasticizing) stage of extrusion, and is retained in the melt in a gaseous or liquid state until the extrudate comes out of the die. This results in the foam structure being very non-uniform. To get a good foam structure, it is preferable to maximize the extraction (purging out) of the moisture from the wood fiber before the final processing of the composite foam in the extruder. However, even oven-dried wood-fiber releases additional moisture or other volatile gases when it is further heated to the processing temperature of the plastics, which is typically above 180° C. This has been shown in a thermo gravimetric analysis (TGA) study [22]. The fibers were initially heated at 110° C. for about 150 minutes so that they were completely dry. At this point, the temperature was raised to 205° C., and it was observed that the wood-fibers lost an additional 3% weight. Therefore it is not sufficient to define the moisture content in terms of the conventional moisture content formula used for lumber wood. This additional moisture, or volatile gases should be taken into account for foaming applications.
A thorough review of the extrusion process is provided by Rauwendaal [6]. The processing of wood-fiber composites is a well-known art, and is described in many patents.
There are a number of patents [7-9] directed to producing wood fiber/plastic composite pellets in which the fibers are “encapsulated” by the plastic. The encapsulation is achieved by processing the mixture at a temperature above the melting point of the plastic. The lowest temperature at which thorough “wetting” occurs is designated as the “encapsulation point”. These pellets are then processed in extruder to make the finished product profile. However, the major draw back with it is that it is batch processing and would involve additional costs associated with such processes.
Turk et al [10], describes a process for preparing a natural fiber and thermoplastic composite, in which they used a vented extruder to remove the gaseous products and moisture released by the fibers. They describe a number of modifications carried out to prevent the loss of the extrudate from the vent. Even after all the modifications, an “interfacial agent” was required for overcoming this problem. This suggests that different materials with different compositions may require different “Interfacial agent”. Therefore, for many compositions suitable “interfacial agent” may not exist.
Deaner et al [11-14] first produced pellets of plastic wood-fiber composites, which were subsequently used for profile extrusion. They describe the importance of reducing the moisture content from the wood-fiber at the pelletizing stage. The moisture in wood fiber is reduced to less than 8 wt-% at this stage. At the final extrusion stage, the moisture is again removed using a vent under 3 inches of vacuum. The final product produced using this process is, however, heavier than the wood it replaces.
Pelikan et al [15] describe a process of producing cellular cavities in which the bubble-forming agent is bound to a carrier, such as vegetable or wood fibers, by molecular or capillary action. In this case the blowing agent is water, it is released at a temperature higher than 120 C., and is thus released more gradually leading to a uniform distribution of closed cells within the plastic matrix.
Boutillier [16] and Cope [17, 18] describe production of plastic/wood-fiber composite foamed profiles using the so-called Celuka process. The outer surface of the extrudate is cooled, below the softening temperature of extrudate, upon exiting from the die. The solidified skin prevents the expansion in the outward direction and the material expands inwards, into a hollow cavity created by a solid body inside the die. However, it is difficult to control the expansion process and the foam structure obtained is non-uniform.
Accordingly, there is clearly a need for foamed thermoplastic/wood-fiber composites, which exhibit fine-celled structure and have easy processability. As mentioned earlier, the removal of moisture from the wood-fiber is necessary to obtain controlled expansion and uniform distribution of the cell structure throughout the product. Further, it should be removed at the highest processing temperature or as close to this temperature as possible. One of the fundamental problems is that the moisture and volatiles from even the dried wood-fibers are typically gradually released, as long as it is being heated. No matter how much it has been devolatilized, these volatiles typically contribute to the foaming of plastic/wood-fiber composites. Accordingly, it would be advantageous to minimize the effect of the moisture and temperature release by minimizing the H2O component at the foaming stage.